1. Field of the Invention
The present invention relates to an on-demand type ink-jet recording method and apparatus in which a black pigment ink and a thick color pigment ink as well as another thin color pigment ink having the same hue are ejected according to printing data to form colored pixels on a recording medium, thereby conducting recording.
2. Related Background Art
An ink-jet recording system is a system in which a single color ink or inks of plural colors corresponding to color recording are ejected on a recording medium (paper, cloth, OHP paper, printed circuit board or the like), thereby forming an image. An ink-jet recording apparatus, to which this system is applied, is equipped with a carriage on which a recording head and an ink tank are mounted, a conveyance part for conveying a recording medium and a control part for controlling them.
In such an ink-jet recording apparatus, ink droplets are ejected from a plurality of ink-jet recording nozzles (hereinafter referred to as “nozzles” merely) while serially scanning the recording head in a direction (main scanning direction) perpendicular to a conveying direction (sub scanning direction) of the recording medium. On the other hand, the recording medium is intermittently conveyed by a quantity equal to a recording width while recording operation is not carried out, thereby successively conducting recording on the recording medium. This recording system has already been widely used as a high-speed, simple and cheap recording system because an ink is directly ejected on the recording medium according to recording signals to conduct recording. In addition, its bright future is expected because it has such a merit that recording can also be conducted on plain paper.
Besides, recording heads for plural colors are provided, whereby such apparatus can easily cope with the formation of a full-color image or high-quality image, so-called photo-image. In the case of such an ink-jet recording apparatus, six recording heads and ink tanks corresponding to six colors in total of black (B), yellow (Y), thick magenta (M) and thick cyan (C) as well as photo-magenta (PM; thin magenta) and photo-cyan (PC; thin cyan) are generally used.
Incidentally, in the conventional ink-jet recording apparatus, those obtained by dissolving various kinds of water-soluble dyes in water or a mixture of water and an organic solvent are used as inks. When the water-soluble dyes are used, however, a problem often arises on light fastness of the resulting recorded image because these water-soluble dyes are naturally poor in light fastness.
The fading phenomenon of ink is known to be caused by photolysis or peroxidation of a color molecule contained in the ink. An oxygen molecule is considered to accelerate the photochemical reaction in the presence of oxygen. This reaction takes place through stages represented by the following formulae (1) to (4) (set forth in P. Douglas, M. Towsend and R. Persico, J. Am. Chem. Soc., 105, 3388 (1983), and the like).                                                        1                    ⁢          DYE                ⁢                  ⟶          hv                ⁢                              DYE                        *                                        1                    ⁡                      (                          Excitation              ⁢                                                           ⁢              of              ⁢                                                           ⁢              color              ⁢                                                           ⁢              molecule                        )                                              (        1        )             1DYE*→3DYE* (Intersystem crossing)  (2)3DYE*+3O2→1DYE+1O2*  (3)(Excitation of oxygen molecule by color molecule)1DYE+1O2*→Photolysis or peroxidation  (4)wherein “DYE” denotes a dye, “1” and “3” respectively represent a singlet and triplet states, and “*” means an excitation state.
Accordingly, when the color molecule is isolated from the oxygen molecule as much as possible, the photolysis or peroxidation of the color molecule is avoided, so that the ink is expected to improve the light fastness.
For example, a pigment with color molecules aggregated is low in proportion coming into contact with oxygen molecules and relatively high in light fastness, so that many ink-jet recording apparatus making use of pigments have been proposed in recent years. For example, Japanese Patent Application Laid-Open No. 56-147871 has proposed water-based pigment inks with a pigment dispersed with a polymeric dispersing agent.
However, the pigment-containing ink, i.e. a pigment ink, has a possibility that the pigment may precipitate on the bottom of an ink tank during long-term storage, or record density may vary according to changes in the concentration of the ink. In addition, there have been fears that clogging would occur in an ink flow path or within nozzles to cause ejection failure of the ink, or in the case of a thermal ink-jet recording apparatus in which thermal energy is used to form a bubble in an ink, and the ink is ejected on the basis of the formation of the bubble, inconveniences such as kogation would occur during ejection of the ink.
In such thin inks of photo-magenta, photo-cyan, etc. as described above, there has been a fear of markedly deteriorating light fastness though their shelf stability is excellent.